Clearly not a significant slice of the electorate (2020 exit polls indicated that he won only about 6% of self-identified Republicans, whereas Hillary Clinton won about 8% of self-identified Republicans in 2016).
I think relatively few Never Trumpers still identify as Republicans.
I also think a lot of these people have become negatively polarized into being very moderate liberals to some extent.
Yeah, unfortunately seems true. The Biden Administration has sent some of them back, but not most, and it seems like there are huge regional differences.